staging: greybus: audio: Avoid less than zero check for le32 variable

mixer control->info call back function checks for -ve values to rebase
min and max values. However, le32 variable is used to fetch values from
GB module FW. Thus negative value checking is not required. Fix this!!

Signed-off-by: Vaibhav Agarwal <vaibhav.sr@gmail.com>
Acked-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>